A handover can be defined as a change of access point in the communication network. In other words, in the handover, a terminal device such as a mobile terminal switches from a cell of a current serving network element such as a central node or a base station to another cell served by another network element. Alternatively, the handover may comprise a change of system, i.e., from the universal mobile terrestrial system (UMTS) to the global system for mobile communications (GSM). The handover typically occurs when the received signal level or another quality metric from another central node is higher than that from the current serving central node. Hence, in order to ensure the required quality of service (QoS), the terminal device may change the serving central node to the one with the highest received power level.
Relay stations or relay nodes have been introduced as a way to extend the coverage area of a central node. Furthermore, they can be applied to reduce the average radio transmission power at the terminal device and to increase the capacity/throughput at the cell-edge. The capacity may also be increased at the shadowed areas in the cell as well as in the locations where the traffic demand is high such as in airports or other hot spots. They may be applied in an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). The E-UTRAN is also known as 3.9G or a Long Term Evolution (LTE) in 3rd generation partnership project (3GPP).
The introduction of the relay nodes to the communication network changes the architecture of the network and increases the probability of the handovers. A system without relay nodes comprises only one type of handover, i.e., from one central node to another. In the communication network with relay nodes, the handover may additionally occur between a central node and a relay node within the same cell, between a central node and a relay node in an adjacent cell, between two relay nodes within the same cell and between two relay nodes in different cells.
Currently there are handover procedures for communication systems with a relay extension. However, in the current handover procedures with the relay extension, the handover is controlled completely at the central nodes. This kind of handover control conducted solely at the central nodes is not flexible and needs improvement.